It is well known that several pathological conditions may be identified by analysing the gases present in blood. The techniques commonly used for these analyses require taking blood samples through various methods and the subsequent storing of these samples in environments that are isolated, thermostated, etc., until the time of the actual analysis. This has various drawbacks well known to those skilled in the art, as well as the impossibility of carrying out a continuous monitoring of the tension of the various gases present in blood. In order to overcome such drawbacks it has been already suggested to dispense with the taking of blood samples and to carry out the determination of the gases present in blood through another way, such as for instance through a transcutaneous way or by analysing saliva samples. These techniques, in addition to being non-invasive, also allow a continuous monitoring of blood gases and the technique for sampling the gases through transcutaneous way in particular has been employed since the beginning in the prenatal diagnostics for determining oxygen and CO2 present in blood.
Apparatuses for analysing blood gases are known, generally comprised of gas sampling probes connected through pipings to apparatuses provided with sensors for measuring the gases. Numerous sensors for analysing blood gases are known, e.g. based on measuring galvanic cells that allow to measure the concentration of one or more gases.
U.S. Pat. No. 5,007,424, e.g., describes a polarographic/amperometric sensor for measuring the oxygen partial pressure in blood by means of a Clark-type electrode arrangement. The sensor may be provided with a pH electrode for the simultaneous determination of CO2 partial pressure in blood.
U.S. Pat. No. 4,840,179 discloses a thermostated device for the simultaneous and continuous measurement of oxygen and CO2 present in blood, based on the principle of pH measurement in an electrolyte. The gas sampling is carried out transcutaneously. However, in order to ensure satisfactory measurements of oxygen and CO2, it is necessary to heat the skin at temperatures of about 42° C. in order to enhance its permeability and consequently the flow of gas.
A problem of galvanic sensors known in the art is that they do not allow to detect the presence of traces of blood gases (such as ammonia, hydrogen sulfide and nitrogen monoxide), which may be related to several pathological conditions. In particular, the gaseous ammonia present in blood may reveal liver and kidney dysfunctions, in which the concentrations increase beyond the physiological values of 0.1-0.6 ppm.
The measurement and the monitoring of gaseous ammonia could allow a rapid and sure diagnosis of diseases like hyperammoniaemia and hypoammoniaemia, diabetes and hypertension, as well as the diagnosis of infection from Helicobacter Pylori. The transcutaneous determination of gaseous ammonia could also be used in haemodialysis treatments and in periodic check-ups.
In the article “Identification of ammonia in gas emanated from human skin and its correlation with that in blood” by K. Nose et al., published on Analytical Sciences, December 2005, vol. 21, page 1471 and following, there is described an experimental study through which it has been possible to detect the presence of gaseous ammonia emanated from the skin and to measure its amount. The article underlines the need for collecting the gases transcutaneously by using methods that are non-painful for the patient and in real time, thus allowing to continuously monitor the variations of gaseous ammonia in blood, as well as to make measuring apparatuses also for domestic use.